2006-08-06
MW Regenerative Receiver
This radio is based on the Moorabbin Receiver. The Regenerative detector is basically identical, however the audio stage in the original radio is dependant on a transformer that is now quite expensive in Australia. I also believe the gain of the original audio stage is insufficient for comfortable listening with weak signals.
The audio pre-amp stage is biased to 750 uA. It is somewhat noisy. Metal film resistors and a transistor with lower noise and better gain at a lower collector current would perform better. (Try a BC550 or BC549C.) However, it is simple and works quite well. You can increase the 10n capacitor in the collector to roll-off the HF response earlier if you find the audio a little tinny. (Doing so will also kill off much of the noise the stage produces.)
The audio "power-amp" is a simple class A affair, biased to 3.5 mA standing current. The DC passes through the speakers, this might be considered undesirable, but is safe at such a low level. Like all such primitive circuits it suffers from positive-going amplitude distortion as the beta of the device changes with its collector current. I experimented with a current mirror to prevent this, the distortion was almost completely eliminated, but at the expense of two more transistors and more than twice the supply current draw. As this device is powered by a 9V battery and I wanted an extremely simple device I elected to use the simpler output stage. The distortion is quite small at "normal" listening levels and is basically a non-issue.
You might like to use something similar to the audio output stage from the VHF regenerative receiver instead.
The device was built into 250 ml Decor brand Polypropylene kitchen container with a friction-fit lid.
The circuit was built point-to-point on a sample swatch of Formica (the kind you find in hardware stores), superglue was used to glue down the 3.5 mm stereo headphone jack, the transistors, and several capacitors, giving it sufficient rigidity. The ferrite rod (Jaycar's old short one) friction-fits inside the top of the box surprisingly well. The tuning and regeneration control caps, and the power switch are mounted on the removable lid. The headphone jack and its attached Formica board bolts to the box side, with some foam padding under the board, securing it in place. The 9v battery is simple held in place with a small block of foam when the lid is closed.
Two banana jacks form the external antenna connections, this was done almost as an after though, the radio is more than sensitive enough for local listening with just the loopstick - in fact with an external antenna you may need to modify the AF stage to have a volume control, reducing the regeneration to control the audio volume compromises the selectivity, which is especially important when you have strong signals like an external antenna can give.
The completed radio pulls about 4.5 mA and will work fairly well down to less than 6 volts. Your average 9 V battery should run the radio for 48 hours or more. The supply decoupling networks are mandatory if you want good stability with higher impedance supplies (like flat 9 V batteries!). The biasing of the AF stages can be modified to work virtually right down to 1.5 V, but at much less than 4.5 V and performance of the RF stage drops. It is definitely possible to make the radio run of a single alkaline cell, but two or more are easier to work with.
Usage Experience
The radio really needs a true RF AGC for casual listening. It is a good radio for beginners to build, it is easy to get going, simple and fun. However, compared to an equally simple MK484-based circuit, its very unpleasant to use pedestrian mobile. On foot or on a bus the huge variations in signal strength leave you playing the regeneration control. There is a particular spot on George St Sydney that 2BL 702 kHz absolutely blasts in at (right at World Square). I don't know why, perhaps it has something to do with the height of the world square building, it may be resonant?
The radio is completely unshielded. Mobile phone radiation goes straight through the receiver and blasts you in the ears. Its quite painful when someone calls you and the mobile in your pocket starts actively radiating. I suspect some careful filtering of the headphone line would help a lot. However, I've noticed that the amount of RF noise detected changes as you tune the radio, probably the internal wiring being tuned as the main gang capacitance changes. It is extremely annoying and makes the radio basically unusable on the Ferry in the afternoon were lots of people are calling their partners to arrange dinner, etc.
TV transmitters, CFL and RFL bulbs, naval radars, repeaters, computers, in fact just about everything electronic radiates RF that will get into this radio and be detected if you aren't careful. This is however quite interesting, listening to all this RF smog is instructive, if at times a little deafening.
A little shielding would go a long way. Peter Parker warns against using a metal box, however if I built the radio again I'd definitely use a Aluminium box. The loopstick could be placed outside, in a piece of Aluminium pipe with a slot cut in it to form an electrostatic shield, but not a shorted-turn. Alternatively the entire box could be slotted and the loopstick mounted inside.
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Attachments
| title | type | size |
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| Circuit Diagram Source | application/postscript | 18.045 kbytes |